This invention pertains generally to systems, devices, and methods for polishing and planarizing substrates, and more particularly to a Chemical Mechanical Planarization or Polishing (CMP) apparatus and method.
Chemical Mechanical Planarization or Polishing, commonly referred to as CMP, is a method of planarizing or polishing semiconductor and other types of substrates. Planarizing a surface of a semiconductor substrate or wafer between certain processing steps allows more circuit layers to be built vertically onto a device. As feature size decreases, density increases, and the size of the semiconductor wafer increase, CMP process requirements become more stringent. Wafer to wafer process uniformity as well as uniformity of planarization across the surface of a wafer are important issues from the standpoint of producing semiconductor products at a low cost. As the size of structures or features on the semiconductor wafer surface have been reduced to smaller and smaller sizes, now typically about 0.2 microns, the problems associated with non-uniform planarization have increased. This problem is sometimes referred to as a Within Wafer Non-Uniformity (WIWNU) problem.
Many reasons are known in the art to contribute to uniformity problems. These include the manner in which wafer backside pressure is applied to the wafer during planarization, edge effect non-uniformities arising from the typically different interaction between the polishing pad at the edge of the wafer as compared to at the central region, and non-uniform deposition of metal and/or oxide layers to might desirably be compensated for by planarizing or adjusting the material removal profile during polishing. Efforts to simultaneously solve these problems have not heretofore been completely successful.
With respect to the nature of the wafer backside polishing pressure, conventional machines typically use hard backed polishing heads to press the wafer against a polishing surface, that is heads having a hard receiving surface that presses directly against the backside of the semiconductor wafer. As a result any variation in the receiving surface of the head, or the presence of any material trapped between the wafer and the receiving surface results in a non-uniform application of pressure to the backside of the wafer. Thus, the front surface of the wafer typically does not conform to the polishing surface resulting in planarization non-uniformities. Moreover, such hard backed head designs often must utilize a relatively high polishing pressure (for example, pressure in the range between about 6 psi and about 8 psi) to provide any reasonable degree of conformity between the wafer and the polishing surface. Such relatively high pressures effectively deform the wafer causing too much material to be removed from some areas of the wafer will be removed and too little material from others resulting in bad planarization.
Attempts have been made to remedy the above problems with hard backed heads by providing an insert between the receiving surface and the wafer to be polished in an attempt to provide some softness in an otherwise hard backed system. This insert is frequently referred to as the wafer insert. These inserts are problematic because they frequently result in process variation leading to wafer-to-wafer variation. This variation is not constant or generally deterministic. One element of the variation is the absorption of water or other fluids such as slurry used in the polishing process. Because the amount of water absorbed by the insert tends to increase over its lifetime, there is frequently process variation from wafer-to-wafer. These process variations may be controlled to a limited extend by preconditioning the insert by soaking the insert in water prior to use and by replacing the insert before its characteristics change beyond acceptable limits. This tends to make the initial period of use more like the later period of use, however, this can increase equipment maintenance costs and decrease process throughput. Moreover, unacceptable process variations are still observed due to, for example, variations in the thickness of the insert, wrinkling of the insert and material being trapped between the hard backed head and the insert or the insert and the wafer.
Use of the insert has also required fine control of the entire surface to which the insert is adhered as any non-uniformity, imperfection, or deviation from planarity or parallelism of the head surface would typically be manifested as planarization variations across the wafer surface. For example, in conventional heads, an aluminum or ceramic plate is fabricated, then lapped and polished before installation in the head. Such fabrication increases the costs of the head and of the machine, particularly if multiple heads are provided.
On the other hand, when a soft backed head is used, the soft material of the insert does not distort the wafer as the wafer is pressed against the polishing pad. As a result, lower polishing pressures may be employed, and conformity of the wafer front surface to the polishing pad is achieved without distortion so that both polishing uniformity and good planarization may be achieved. Better planarization uniformity is achieved at least in part because the polishing rate on similar features from die to die on the wafer is the same.
In recent years, some attempts have been made to utilize soft backed heads, however, they have not been entirely satisfactory. One type of soft backed head is described in U.S. Pat. No. 6,019,671, to Shendon, hereby incorporated by reference. Shendon teaches a membrane or flexible member stretched across the lower surface of the head to form a chamber or cavity which is pressurized to press the substrate against the polishing surface. While a significant improvement over hard backed heads with or without inserts this approach is not wholly satisfactory for a number of reasons. One problem with this approach is that it does nothing to reduce or eliminate the non-uniformities due to material trapped between the membrane and the wafer. Another problem is the membrane prevents the use of vacuum to hold the wafer to the head during a load or unload operation. Moreover, the use of the membrane can actually increase non-uniformities by introducing new variables, such as variation in the thickness or flexibility of the membrane across its surface and possible wrinkling of an improperly installed membrane.
Other soft backed head designs use a seal between the edge of the wafer and the head to form a cavity which is then pressurized to directly press the wafer against the polishing surface during polishing and planarization. One approach is described in U.S. Pat. No. 5,635,083, to Breivogel, et al., hereby incorporated by reference. Breivogel teaches the use of a lip seal against the outer edge of the backside of said wafer to form a seal between the head and the wafer to which pressurized air is admitted. Unfortunately, while such an approach provides a soft backed head that eliminates some of the problems associated with hard backed heads and soft backed heads having membranes, it does not permit sufficient engagement between the wafer and the receiving surface to provide torque to the wafer in machines where the head rotates during the polishing operation. Another problem with this approach is that although vacuum can be used to hold the wafer to the head, because the wafer is supported only at the edge an unacceptable degree of bowing can occur resulting in damage to or loss of the wafer.
With respect to correction or compensation for edge polishing effects, attempts have been made to adjust the shape of the retaining ring and to modify a retaining ring pressure so that the amount of material removed from the wafer near the retaining ring is modified. Typically, more material is removed from the edge of the wafer, that is the wafer edge is over polished. In order to correct this over polishing, usually, the retaining ring pressure is adjusted to be somewhat higher than the wafer backside pressure so that the polishing pad in that area is somewhat compressed by the retaining ring and less material is removed from the wafer within a few millimeters of the retaining ring. However, even these attempts are not entirely satisfactory as the planarization pressure at the outer peripheral edge of the wafer is only indirectly adjustable based on the retaining ring pressure. It is not possible to extend the effective distance of a retaining ring compensation effect an arbitrary distance into the wafer edge. Neither is it possible to independently adjust the retaining ring pressure, edge pressure, or overall backside wafer pressure to achieve a desired result.
Another problem with the retaining ring in conventional CMP heads is that any given point on the lower surface of the retaining ring corresponds to a given part of a wafer held on the subcarrier throughout the polishing operation. Thus, high or low spot on the lower surface of the retaining ring will result in non-planar polishing of the wafer. Although, it is possible to machine the lower surface of the retaining ring to have a high degree of flatness this is a costly option, especially since retaining rings are consumable components that wear as the wafer is polished and must frequently be replaced.
With respect to the desirability to adjust the material removal profile to adjust for incoming wafer non-uniform depositions, few if any attempts have been made to provide method or machines that afford such compensation. Non-uniform depositions can arise from the structure of circuits formed on the wafer or from characteristics of the deposited layers. For example, copper layers, which have become increasingly common in high-speed integrated circuits tend to form a convex layer thicker at the center of the wafer than the edge. Thus, it would be desirable to have a polishing method and an apparatus that provided a higher removal rate near the center of the wafer than at the edge.
A final problem with conventional CMP apparatuses and methods is the inefficient use and wastage of slurry. Slurry is a, usually, chemically active liquid having an abrasive material suspended therein that is used to enhance the rate at which material is removed from the substrate surface. Because the slurry is dispensed onto the polishing surface ahead of the head, an excess of slurry must typically be dispensed to ensure that when it flows across the polishing surface it will cover the entire area between the wafer and the surface. Because of strict requirements concerning the purity of the slurry and in particular the size of the abrasive particles suspended therein, slurry tends to be expensive. Moreover, to avoid contamination and to provide consistent results slurry is generally not recirculated or recycled. Thus, a significant factor in the cost of operating conventional CMP apparatuses is the cost of the slurry.
Therefore, there remains a need for an apparatus and method that provides excellent planarization, controls edge planarization effects, and permits adjustment the wafer material removal profile to compensate for non-uniform deposition of layers on the wafer. There is a further need for an apparatus and method that enables the wafer to be held to the head by vacuum to a soft backed head while minimizing or eliminating stresses on the wafer. There is yet a further need for a CMP apparatus that provides a sufficient slurry to the polishing surface without excessive amount of wastage.
The present invention relates to a CMP apparatus and method for polishing and planarizing substrates that achieves a high-planarization uniformity across the surface of the substrate, while providing a more efficient use of slurry in the polishing and planarizing processes.
According to one aspect of the present invention, polishing head for positioning a substrate having a surface on a polishing surface of a polishing apparatus is provided for processing the substrate to remove material therefrom. The polishing head includes a carrier, a subcarrier carried by the carrier and adapted to hold the substrate during a polishing operation and a retaining ring rotatably disposed about the subcarrier. The retaining ring has a lower surface substantially flush with a surface of a substrate held on the subcarrier, the lower surface of the retaining ring in contact with the polishing surface during processing. The retaining ring deforms the polishing surface to reduce the rate of removal of material from the edge of the substrate. With conventional carriers any variation or irregularity in the lower surface of the retaining ring cause a locally higher or lower removal rate of material from the edge of the substrate adjacent to the irregularity. However, with a carrier of the present invention, because the retaining ring is able to rotate relative to the subcarrier, and therefore the substrate, during processing, the impact of any variation in the lower surface of the retaining ring on the rate of removal of material from the edge of the substrate is minimized.
In one embodiment, the subcarrier is driven by a drive mechanism, and friction forces between the retaining ring and the polishing surface causes the retaining ring to rotate relative to the subcarrier. Alternatively, the retaining ring can be caused to rotate relative to the subcarrier by a separate drive mechanism coupled to the retaining ring.
In another embodiment, the carrier further includes a backing ring in a facing relationship with an upper surface of the retaining ring and separated from the retaining ring by a bearing. The backing ring is adapted to apply pressure to the retaining ring during the polishing operation. The bearing can be, for example, a ball bearing, fluid dynamic bearing, roller bearing or a taper bearing. Optionally, the retaining ring further includes a first lip that engages with a second lip on the backing ring when the carrier is lifted from the polishing surface to couple the retaining ring to the backing ring.